you know those pictures where everyone has the same face? where can I go to do that, that's free??

I'm doing an ad buster for english, and I have this american eagle as with alot of people in it and I want to do the face duplication on all of them and instead "live your life" I'm going to put "live the same life" or "live your friends life" <<<any ideas for that part, FEEL FREE (: